The present invention relates generally to hydrogen and/or synthesis gas production and more specifically to determination of the carbon content of a hydrocarbon-containing feedstock for a hydrogen and/or synthesis gas production process.
A feedstock for a hydrogen and/or synthesis gas production process, for example steam methane reforming (SMR), autothermal reforming (ATR), and catalytic partial oxidation (CPOX), is generally a hydrocarbon-containing mixture such as natural gas and may include refinery offgas. One of the properties of the hydrocarbon-containing mixture is its carbon content. The carbon content is used to determine the amount of steam to be combined with the hydrocarbon-containing mixture to form a mixed feed before introducing the mixed feed into a hydrogen-forming reactor.
The amount of steam to be combined with the hydrocarbon-containing mixture is determined by a control parameter typically called the steam-to-carbon ratio. Accurate determination of the carbon content of the hydrocarbon-containing mixture is important to the hydrogen-forming process so that a suitable amount of steam is added. An insufficient amount of steam will lead to carbon formation on the catalyst in the hydrogen-forming reactor, thereby resulting in carbon deposition and degrading the activity of the catalyst, while too much steam decreases the energy efficiency of the process.
Commonly, the carbon content is determined by analyzing the composition of the hydrocarbon-containing mixture. Typically, the composition is analyzed by mass spectroscopy or gas chromatography, which are costly and require frequent maintenance. Further, gas chromatography provides results with a time delay, so that any abrupt change in the composition is not detected until after hydrocarbon-containing mixture has already been introduced into the hydrogen-forming process.
Therefore, due to the time delay in the measurements, the steam-to-carbon ratio is often set conservatively to provide more steam than needed by the hydrogen-forming process. In this way, carbon formation on the catalyst can be avoided at the expense of the energy efficiency.
It would be desirable to provide carbon content measurements with low cost and low maintenance, preferably with little or no time delay.
To avoid the cost and maintenance of compositional analysis, attempts have been made to use approaches that do not use compositional analysis.
One approach is to assume a fixed carbon content for the hydrocarbon-containing mixture, for example when the composition varies within a small range, such as when the hydrocarbon-containing mixture is natural gas. Since underestimating the carbon content could lead to carbon formation on the catalyst, the assumed carbon content is chosen conservatively based on the historical composition of the hydrocarbon-containing mixture as determined by off-line composition analysis. Therefore the steam-to-carbon ratio is generally greater than required, resulting in a reduced energy efficiency of the hydrogen-forming process. The catalyst is also at risk if the carbon content of the hydrocarbon-containing mixture increases above its historical range.
Another approach is based on measuring density or molecular weight of the hydrocarbon-containing mixture. Sensors for measuring density or molecular weight are usually less expensive and require less maintenance than mass spectrometers and gas chromatographs.
A correlation between the carbon content of a hydrocarbon-containing mixture and density or molecular weight can be established based on the historical composition data of the hydrocarbon-containing mixture. This approach is more suitable than using a fixed carbon content for the hydrocarbon-containing mixture. A change in the carbon content due to a change in the proportion of different hydrocarbons will cause a change in the density or molecular weight, and the correlation will use this change to provide a more accurate estimate of the carbon content of the hydrocarbon-containing mixture.
FIG. 1 shows molecular weight plotted as a function of carbon number for straight chain alkanes, methane through hexane, nitrogen, carbon monoxide, carbon dioxide and hydrogen. FIG. 1 shows that there is a good linear correlation between molecular weight and the carbon number for alkanes. Hydrogen can also be included with the alkanes in the correlation. Because of the linear relationship, when these alkanes and hydrogen form a mixture, a measurement of the molecular weight is useful for determining the carbon number.
Since the accuracy of the calculated carbon number depends on the accuracy of the molecular weight, it would be desirable to provide accurate measurement of the molecular weight.
A problem with using density or molecular weight is that the density and molecular weight also depend on the non-hydrocarbon components in the hydrocarbon-containing mixture, for example nitrogen, argon, carbon dioxide, carbon monoxide and water. If the amount of these non-hydrocarbon components varies significantly, it will cause greater error in the carbon content calculated by the correlation. If the amount of non-hydrocarbon components goes outside the historical range where the correlation was developed, the correlation may become very unreliable.
It would be desirable to provide accurate carbon content values so that the steam-to-carbon ratio can be set less conservatively, thereby preventing carbon formation on the catalyst and improving energy efficiency.
The present method solves the long felt need for determining the carbon content of a hydrocarbon-containing mixture with improved accuracy and response time, thereby allowing less conservative steam-to-carbon ratios to be targeted.
Related disclosures include Japanese Patent Application No. 08-291195, U.S. Pat. No. 6,758,101 and European Pat. Application EP 1,213,566.